Why the square brackets? If those are intended to be single character bracket expressions, tr does not support that syntax. Well, I should say, POSIX tr. Perhaps that is an older dialect? In modern implementations, that simply replaces [ with itself and ] with itself, a couple of no-ops, in addition to comma with tab.

Obviously harmless if the square brackets are both sides. I had never noticed that unescaped [] were not treated as special characters in some syntaxes of Posix "tr".
Strangely I don't usually put [] for single characters but it ended up like that after simplifying the command.

The "tr" command line modifies the default translation table for that locale. There is no additional processing when the translate itself takes place.